1. Technical Field
The present application relates generally to the field of thin film resistors, both in integrated circuits and as discrete resistors, more specifically to thin film resistors with improved temperature independence, and even more specifically to thin film resistors fabricated from carbon nanotube fabrics.
2. Discussion of Related Art
Thin film resistors are used in many important technological applications as parts of electronic circuits. Thin film resistors may be integrated into complex hybrid circuitry, and/or they may be used as discrete devices. For example, thin film resistors can be used in integrated circuits as the resistive ladder network in an analog-to-digital converter, and as current limiting and load resistors in emitter follower amplifiers.
Currently, thin film resistors are fabricated using a variety of materials including tantalum nitride (TaN), silicon chromium (SiCr) and nickel chromium (NiCr).
In general, the resistance of a material changes with temperature; this property can be quantified as the temperature coefficient of resistance (TCR) for a given material. For example, as the material of a resistor is heated, the resistance of the material may rise. For example, the TCR of NiCr thin film resistors is on the order of 120 ppm/degree C. This relatively large TCR means that a small temperature variation across a data converter could detune the device, e.g., change a specifically tuned resistance value of the device. When tuning the resistance of a device is imperative, then the TCR must be kept to a minimum.
Some presently available high performance thin-film resistors are capable of exhibiting low TCRs, but their applicability is limited because they are not able to carry large currents. Currently, high performance thin film metal resistors are approximately 1-5 atoms thick and have sheet resistances measuring approximately 1-2 K-Ohms per square. Such resistors can support a current of up to approximately 20 u-Amps /micron width. Large resistance values in excess of 100 K-Ohms are difficult to design are difficult to design and fabricate.
Thin film resistors, typically on a substrate (carrier), may be backside mounted, that is, they may be mechanically attached to the next level of electronic assembly, and may be wirebonded for electrical interconnection. Typically, wire bond electrical connections have a low resistance in the milliohm range and an inductance of 1.5 to 2 nanoHenries (nH). Alternatively, thin film resistors, typically on a substrate (carrier), may include terminal metallurgy and a conductive bump, and may be surface mounted to the next level of assembly using well known flip-chip techniques. The conductive bumps provide both mechanical and electrical connections to the next level of assembly. Typically, electrical connections using conductive bumps have a low resistance in the milliohm range and a low inductance typically less than 0.5 nanoHenries (nH).
Parasitic capacitances and inductances associated with thin film resistor values are a function of the length and width of the thin film resistor. If the thin film resistor is in the proximity of one or more conductive planes (such as a substrate, for example), then parasitic capacitances and inductances associated with the resistor are determined by the length and width of the thin film resistor, and the distance from one or more conductor reference planes. Parasitic capacitance and inductance values are reduced as the thin film resistor size is reduced, thus, higher sheet resistance thin films are more desirable for higher resistance values (10 K-Ohm to 100 K-Ohm and above resistors, for example).
Single walled carbon nanotubes exhibit quasiballistic electron transport at room temperature. This property of nanotubes lends itself to conductivity with very low resistance. See Kong, Jing et al., “Quantum Interference and Ballistic Transmission in Nanotube Electron Waveguides”, Phys. Rev. Lett., 2001, 87 (10) 106801-1-106801-4; Javey, Ali et al., “High-Field Quasiballistic Transport in Short Carbon Nanotubes”, Phys. Rev. Lett., 2004, 92 (10) 106804-1-106804-4; Javey, Ali et al., “Ballistic Carbon Nanotube Field-Effect Transistors”, Nature, 2003, 424, 654-657.
Carbon nanotubes exhibit electrical characteristics appropriate for use in numerous devices. Rueckes et al. have described non-woven conductive fabric made from carbon nanotubes in U.S. Pat. Nos. 6,706,402 and 6,835,591; also see U.S. patent application Ser. Nos. 10/341,005, 10/341,054, 10/341,055 and 10/341,130. Such films are used, for example, as elements in memory cells, see U.S. Pat. No. 6,706,402, and as sensor elements. See U.S. patent application Ser. Nos. 10/844,913 and 10/844,883. Their conductive and semiconductive properties also make them suitable for other uses in the electronics industry.